Outline - Christian Liberty (PDF)
One of Luther's best. He tries to defend two seemingly contradictory propositions: (1) The Christian is perfectly free Lord of all and subject to none, and, (2) the Christian is the dutiful servant of all (his neighbor). In doing this, he does a great job of expositing what Justification is (as opposed to sanctification).
Easy to read and comprehend, written in Luther's inimitable "fire-in-his-pen" style, but intended to comfort tormented souls with the Balm of Gilead, that is Jesus Christ dying for our sin and setting us free from the law's condemnation and Satan's accusations. This is a book that includes Luther's image of "The Great Exchange" between the sinner and Christ.